Acta Limnologica Brasiliensia, 2015, 27(2), 145-159 http://dx.doi.org/10.1590/S2179-975X3414 An overview of the contribution of studies with cladocerans to environmental stress research Um panorama da contribuição de estudos com cladóceros para as pesquisas sobre o estresse ambiental Albert Luiz Suhett1, Jayme Magalhães Santangelo2, Reinaldo Luiz Bozelli3, Christian Eugen Wilhem Steinberg4 and Vinicius Fortes Farjalla3 1Unidade Universitária de Biologia, Centro Universitário Estadual da Zona Oeste – UEZO, CEP 23070-200, Rio de Janeiro, RJ, Brazil e-mail: [email protected] 2Departamento de Ciências Ambientais, Instituto de Florestas, Universidade Federal Rural do Rio de Janeiro – UFRRJ, CEP 23890-000, Seropédica, RJ, Brazil e-mail: [email protected] 3Departamento de Ecologia, Instituto de Biologia, Universidade Federal do Rio de Janeiro – UFRJ, CEP 21941-590, Rio de Janeiro, RJ, Brazil e-mail: [email protected]; [email protected] 4Institute of Biology, Faculty of Mathematics and Natural Sciences I, Humboldt Universität zu Berlin, 12437, Berlin, Germany e-mail: [email protected] Abstract: Cladocerans are microcrustaceans component of the zooplankton in a wide array of aquatic ecosystems. These organisms, in particular the genusDaphnia , have been widely used model organisms in studies ranging from biomedical sciences to ecology. Here, we present an overview of the contribution of studies with cladocerans to understanding the consequences at different levels of biological organization of stress induced by environmental factors. We discuss how some characteristics of cladocerans (e.g., small body size, short life cycles, cyclic parthenogenesis) make them convenient models for such studies, with a particular comparison with other major zooplanktonic taxa. Then we illustrate the contribution of cladocerans to stress research with examples encompassing stress responses spanning from the molecular to the populational level. Most worth of note are recent studies that presented evidence of beneficial consequences of mild stress caused by natural stressors (cross-tolerance), which may be passed along across generations, favoring individual survival and species persistence in fluctuating environments. This would be particularly relevant for environments prone to frequent natural environmental fluctuations, such as coastal lagoons and other shallow aquatic ecosystems. Based on reviewed studies, a conceptual model is presented summarizing the potential effects of a first stressor on the organism’s resistance to a second one. We finish by highlighting some gaps on environmental stress research that could benefit from further studies using cladocerans as model organisms. Keywords: Cladocera; stress; cross-tolerance; ecotoxicology; ecophysiology. Resumo: Os cladóceros são microcrustáceos integrantes do zooplâncton em um vasto espectro de ambientes aquáticos. Estes organismos, em especial o gênero Daphnia, têm sido organismos modelo muito utilizados em estudos abrangendo das ciências biomédicas à Ecologia. No presente trabalho, fazemos uma revisão da contribuição de estudos com cladóceros para o conhecimento sobre as consequências, em diferentes níveis de organização biológica, do estresse induzido por fatores ambientais. Discutimos de que forma características peculiares dos cladóceros (e.g., pequeno tamanho corporal, ciclo de vida curto, partenogênese cíclica) fazem deles modelos bastante convenientes para tais estudos, com uma comparação particular com outros grandes grupos zooplanctônicos. Em seguida, ilustramos a contribuição dos cladóceros para as pesquisas sobre estresse com exemplos englobando respostas ao estresse do nível molecular ao populacional. São ressaltados de forma especial estudos recentes que apresentam evidências de consequências 146 Suhett, A.L. et al. Acta Limnologica Brasiliensia benéficas do estresse brando causado por estressores naturais (tolerância cruzada), as quais podem ser transmitidas entre gerações, favorecendo a sobrevivência e a persistência de espécies em ambientes muito variáveis. Isto seria particularmente relevante para ambientes sujeitos a frequentes variações naturais intensas, tais como lagoas costeiras e outros ecossistemas aquáticos rasos. Com base nos estudos revisados, construímos um modelo conceitual resumindo os potenciais efeitos de um primeiro estressor na resistência dos organismos a um estressor subsequente. Concluímos chamando a atenção para algumas lacunas nas pesquisas sobre o estresse em contexto ambiental, as quais poderiam ser preenchidas por estudos futuros utilizando cladóceros como modelos. Palavras-chave: Cladocera; estresse; tolerância cruzada; ecotoxicologia; ecofisiologia. 1. Introduction making them even more interesting models for research on environmental stress. Among In the ecological literature, particularly within cladocerans, the genus Daphnia receives special Ecophysiology, stress has been a recurrent theme. attention from researchers due to its widespread Organisms are constantly subject to variation in distribution, especially in temperate regions their environment, and many of these varying (Sarma et al., 2005). Daphnia species, particularly factors act as stressors, displacing the organisms Daphnia magna, have become important model from their fundamental niches (Van Straalen, organisms for studies ranging from biomedical 2003; Steinberg, 2011). Besides natural stressors, sciences to ecology, but they share several ecological human-induced environmental changes—among features with other cladocerans that offer similar which, the increasing chemical pollution of advantages (Ebert, 2011; Seda & Petrusek, 2011; ecosystems—represent important sources of stress Miner et al., 2012). in the environment. Most of the understanding of In this review, we aim to present an outlook of the how stress affects organisms comes from studies with model organisms that are small enough to contribution of cladoceran studies to environmental be cultivated and used in short-term experiments stress research. First, we briefly discuss the definition in the laboratory. Among the most popular model of stress in the light of recent, challenging findings organisms in Biology are the Drosophila fruitflies, regarding the effects of mild stress. Then, we present a the clawed frog Xenopus laevis and the nematode brief description of cladoceran biology, highlighting Caenorhabidits elegans (Seda & Petrusek, 2011). some of the reasons for their prevalence over other In aquatic ecosystems, zooplanktonic organisms zooplanktonic groups in studies about stress. We have been widely used as models for studies testing follow with a set of examples of cladoceran studies and developing ecological theories, as reviewed using different approaches (e.g. biochemistry or by Lampert (1997). Among the examples cited life table experiments) to address the consequences therein are studies on mechanistic models of of stress to organisms. We finish by considering competition (Rothhaupt, 1988) and inducible some gaps that may be filled up with the use of phenotypic changes (Taylor & Gabriel, 1992). cladocerans, which could contribute to the advance Short life cycles, small body size and their central of stress research in a pure ecological point of view, role in pelagic food webs are some of the main as well as in the context of ongoing natural and features that make zooplanktonic organisms so human-induced changes experienced by ecosystems. suitable for such studies. One specific group 2. Defining Stress within zooplankton—namely the crustacean order Cladocera—has received special attention across The term stress, much alike other highly ecological studies, in particular those dealing pervasive terms in Ecology—e.g., adaptation, with environmentally-induced stress from its disturbance—may have as many definitions as there many perspectives (e.g., biochemical, molecular, are sub-disciplines and methodological approaches physiological, toxicological). to address it. It is not our aim here to discuss in Cladocera have peculiar biological features depth the definition of stress (for reviews, see that make their use in experimental studies very Parker et al., 1999; Bijlsma & Loeschcke, 2005). appealing. Moreover, cladocerans may be found Nevertheless, we wish to derive a consensual in highly fluctuating environments, such as ponds definition of stress based on recent debates in the and shallow coastal lagoons (Petrusek, 2002; literature, before we proceed to present and discuss Santangelo et al., 2008; Dodson et al., 2009), the reviewed studies. 2015, 27(2), 145-159 An overview of the contribution of studies with cladocerans… 147 Figure 1. Scientometric analysis of the contribution of studies with the major zooplanktonic taxa to the research on stress, highlighting the prevalence of cladocerans over rotifers and copepods in number (a) and percentage (b) of published papers. For details on the methods used for the survey, please refer to the footnote1. Stress is the state experienced by an organism Put this way, stress is seen as a fundamental aspect when it is challenged by abiotic and/or biotic of life itself and, being an ever-acting environmental changes in its environment (stressors), which and evolutionary force, not only at the “harsh stress translate into a set of identifiable symptoms (stress – detrimental effects” range Steinberg et al.,( 2006; responses), mostly at the suborganismal level, e.g. Steinberg, 2011). biochemical and molecular levels (Parker et al., 1999; Van Straalen,